Responses
From what I gather Makoto met Haru in a past life when Haru was a mermaid and they fell in love. The were both reincarnated, Haru as a human boy and Makoto as Santa. Makoto knew about their past life first then Haru remembered (or he at least remembered his past feelings of love). They both fell in love again and Haru wished to see Santa Makoto again but couldn't because he had grown up. I assume Makoto heard this wish and granted it by becoming a human. And then they lived happily ever after in love♡♡♡
but... confusing. so who the heck was Santa???? (⊙…⊙ )